Disk drive memory systems have been used in computers for many years for the storage of digital information. Information is recorded on concentric tracks of a magnetic disk medium, the actual information being stored in the form of magnetic transitions within the medium. The disks themselves are rotatably mounted on a spindle. Information is accessed by a read/write transducer located on a pivoting arm that moves radially over the surface of the rotating disk. The read/write head or transducer must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
During operation, the disks are rotated at very high speeds within an enclosed housing using an electric motor generally located inside a hub or below the disks, for example an in hub or in spindle motor. The spindle includes bearing components to support the rotation and axial location of the disc stack. Such motors may have a spindle mounted by two ball bearing systems to a motor shaft disposed in the center of the hub. However, with the decreasing size of information storage systems, other types of bearings including fluid dynamic bearings are being developed, such as those useful designs discussed herein.
In these types of bearings, a lubricating fluid, i.e., gas, liquid or air is used in the active bearing region to generate fluid dynamic pressure to prevent metal to metal contact.
The bearing region comprises two relatively rotating surfaces, at least one of which supports or has defined thereon pattern of grooves. The grooves collect fluid in the active bearing region. When the two surfaces of the bearing rotate relative to one another, a pressure profile is created in the gap due to hydrodynamic action. This profile establishes a stabilizing force so that the bearing surfaces rotate freely without contact. In a disc drive, the rotating surface is associated with a hub supporting one or more discs whose rotation and axial location is kept stable by the pressure profile.
The tangential forces created in the bearing area characterize the bearing with respect to changes in shear in the fluid and are summed up in torque, which in turn defines power consumption. The pressure profile defines all forces normal to the bearing surface which characterize the bearing with respect to axial load and radial and angular restoring forces and movement.
A specific fluid dynamic bearing design can be characterized by multiple qualities, including power consumption, damping, stiffness, stiffness ratios and restoring forces and moments.
The design of the fluid dynamic bearing and specifically the groove pattern, is adapted to enhance the stiffness and damping of the rotating system, which includes one or more discs rotating at very high speed. Stiffness is the changing force element per changing distance or gap; damping is the change force element per changing rate of distance or gap. Optimizing these measures reduces non-repeatable run out (NRRO), an important measure of disc drive performance.
A further critical issue is the need to maintain the stiffness of the hydrodynamic bearing. The stiffer the bearing, the higher the natural frequencies in the radial and axial direction, so that the more stable is the track of the disc being rotated by a spindle on which reading and writing must occur. Thus the stiffness of the bearing in the absence of any mechanical contact between its relatively rotating parts becomes critical in the design of the bearing so that the rotating load can be stably and accurately supported on the spindle without wobble or tilt.
A typical prior art grooving pattern for fluid dynamic bearings is shown in FIG. 2. A plurality of grooves with constant groove angle along its length can be arranged either on the outer surface of the shaft or on the inner surface of the sleeve. The groove number, angle, groove pitch ratio (GPR), groove depth, etc. can be adjusted to obtain the best stiffness, damping and power to optimize the bearing dynamic performances. However, there are several short comings associated with existing grooving patterns such as that illustrated. For one, fluid flow is mainly limited inside or along the grooves. Second, the grooves do not provide enough restoring moment to protect the bearing from operating shock and impact. Third, the symmetric parts of the grooves are mirror images about their apexes and hence their characteristics are complementary to each other without providing any active control over their performance variations. Fourth, a wide range of pressure fluctuations is generated along a line through the apexes of the successive grooves of a bearing pattern. Fifth, the limited bearing space is not always efficiently used with such grooves to enhance bearing performance. Finally, these grooves require asymmetry to prevent leakage or to have net flow of fluid.
Thus, there is a need in the art for a fluid dynamic bearing grooving pattern that improves one or more of fluid circulation, distribution at the apex and on the bearing surface, restoring moment, control over top and bottom groove sets and pressure fluctuations at the apex. A number of efforts have been made in the prior art to optimize bearing performance by modifying groove depth, width, shape, and/or space. However, it would be advantageous if the different size and shaped grooves could be easily adapted to optimize bearing performances and yet could prevent leakage (in the case of liquids) without requiring asymmetry. This will allow bearings to be spaced apart with longer spans which in turn will increase rocking stiffness and hence restoring moment. It is also desirable to provide additional groove pattern shapes, lengths, and spacing to allow trade-offs between features that qualify a bearing.
Further, in conventional fluid dynamic bearings, the groove pattern is formed on only one of the bearing surfaces to pump fluid and generate pressure. The direction of the grooves will depend on which surface of the bearing they are formed on and the direction of rotor rotation. However, this arrangement does not provide maximized bearing performance. Thus, there exists a need in the art for a fluid dynamic bearing that provides groove patterns on both of the bearing surfaces to generate more pressure and thereby enhance bearing performance.